January 2014 

NASA/TM–2014-218148 

Current Performance Characteristics of NASA 

Langley Research Center’s Cockpit Motion 

Base and Standarized Test Procedure for Future 

Performance Characterization 

Brandon Cowen and Mary T. Stringer 

Adaptive Aerospace Group, Inc., Hampton, Virginia 

Brian K. Hutchinson, Paul C. Davidson, and Lawrence E. Gupton 

Langley Research Center, Hampton, Virginia 
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January 2014 

NASA/TM–2014-218148 

Current Performance Characteristics of NASA 

Langley Research Center’s Cockpit Motion 

Base and Standardized Test Procedure for 

Future Performance Characterization 

Brandon Cowen and Mary T. Stringer 

Adaptive Aerospace Group, Inc., Hampton, Virginia 

Brian K. Hutchinson, Paul C. Davidson, and Lawrence E. Gupton 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320

443-757-5802 

ACKNOWLEDGMENTS 

Special thanks are extended to the many people that contributed to the extensive testing 

and evaluation process during the development of this report. This effort would not have 

been possible without the guidance and assistance of Fred Lallman, Robert Redman, Dale 

Ashcom, and Gustav Taylor of the NASA Langley Research Center, Keith Hoffler of 

Adaptive Aerospace Group, as well as Frank Cardullo of the State University of New 

York- Binghamton. 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not 

constitute an official endorsement, either expressed or implied, of such products or manufacturers by the 

National Aeronautics and Space Administration. 
NOMENCLATURE 

Symbols 

CMB Cockpit Motion Base 

CMF Cockpit Motion Facility 

DCU Digital Control Unit 

DoF Degree-of-Freedom 

DVS Data Visualization System 

GFD Generic Flight Deck 

IFD Integration Flight Deck 

PSD Power Spectral Density 

RFD Research Flight Deck 

SDAB Simulation Development and Analysis Branch 

SID System Identification 

SIMES SIMulator Evaluation System 

iii
Table of Contents 

NOMENCLATURE iii

LIST OF FIGURES v

INTRODUCTION 2

CMB MOTION SYSTEM CONFIGURATION 4

DIGITAL CONTROL LAW 4

CMB Parasitic Accelerations- Past and Present 5

Control Law Modification 7

CMB PERFORMANCE AND COMPARISONS 9

Transport Delay 9

Performance Test Input Signals 10

Performance Characteristics 12

CMF STANDARDIZED TESTING PROTOCOL 21

Standard Spectral Inputs 21

Chirp Inputs 21

Banked, Sinusoidal Inputs 22

Standard Test Procedure 23

Standard Post-Processing Tools 23

Automated Processing Scripts 23

Graphical User Interface (GUI) 23

Additional Functions 24

CONSIDERATIONS 24

CONCLUSIONS 25

BIBLIOGRAPHY 26

APPENDIX 27

Comparison of Production Controller to Previous Controller 27

iv
LIST OF FIGURES 

Figure 1: GFD interior (left) and exterior on CMB (right) .............................................................. 2

Figure 2: CMF representation showing the CMB, available cabs, and the crane ............................ 3

Figure 3: Control architecture for the CMB..................................................................................... 3

Figure 4: Cab-axes coordinate frame ............................................................................................... 4

Figure 5: Uncommanded motion and residual motion for 0.45 inch- 4Hz sinusoid Leg 1 position 

command. ......................................................................................................................................... 6

Figure 6: Uncommanded motion during 1 inch- 2Hz sinusoid level heave command .................... 6

Figure 7: Leg target-velocity test results.......................................................................................... 7

Figure 8: Parasitic accelerations during increasing banked heave oscillations ................................ 8

Figure 9: System latency test using SIMES with 10kHz sampling rate- 20 sec step input to Leg 5.

 ....................................................................................................................................................... 10

Figure 10: Time history of commanded frequency sweep ............................................................. 11

Figure 11: Power Spectral Density of commanded frequency sweep ............................................ 12

Figure 12: Bode Plot for Production Controller - Heave ............................................................... 13

Figure 13: Bode Plot for Production Controller - Sway ................................................................ 13

Figure 14: Bode Plot for Production Controller – Surge ............................................................... 14

Figure 15: Bode Plot for Production Controller – Pitch ................................................................ 14

Figure 16: Bode Plot for Production Controller – Roll .................................................................. 15

Figure 17: Bode Plot for Production Controller – Yaw ................................................................. 15

Figure 18: Production Controller - Parasitic Acceleration from Commanded Heave (dB) ........... 16

Figure 19: Production Controller - Parasitic Acceleration from Commanded Pitch (dB) ............. 17

Figure 20: Production Controller - Parasitic Acceleration from Commanded Roll (dB) .............. 17

Figure 21: Production Controller - Parasitic Acceleration from Commanded Surge (dB) ............ 18

Figure 22: Production Controller - Parasitic Acceleration from Commanded Yaw (dB) .............. 18

Figure 23: Production Controller - Parasitic Acceleration from Commanded Sway (dB) ............ 19

Figure 24: Operational Limits (Heave) .......................................................................................... 20

Figure 25: Operational Limits (Sway) ........................................................................................... 20

Figure 26: Operational Limits (Banked Heave) ............................................................................. 21

Figure 27: Sample sweave-spline time history .............................................................................. 22

Figure 28: GUI - Post Processing Tool .......................................................................................... 24

Figure 29: CMB valve performance .............................................................................................. 25

Figure 30:Bode Plot Comparison – Heave .................................................................................... 27

Figure 31: Bode Plot Comparison – Sway ..................................................................................... 27

v
Figure 32: Bode Plot Comparison – Surge .................................................................................... 28

Figure 33: Bode Plot Comparison – Pitch ..................................................................................... 28

Figure 34: Bode Plot Comparison – Roll ....................................................................................... 29

Figure 35: Bode Plot Comparison – Yaw ...................................................................................... 29

Figure 36: Previous Controller - Parasitic Acceleration from Commanded Heave (dB) ............... 30

Figure 37: Previous Controller - Parasitic Acceleration from Commanded Pitch (dB) ................. 30

Figure 38: Previous Controller - Parasitic Acceleration from Commanded Roll (dB) .................. 31

Figure 39: Previous Controller - Parasitic Acceleration from Commanded Surge (dB) ............... 31

Figure 40: Previous Controller - Parasitic Acceleration from Commanded Sway (dB) ................ 32

Figure 41: Previous Controller - Parasitic Acceleration from Commanded Yaw (dB) ................. 32

Figure 42: Previous Controller – Cross-coupled acceleration gains by increasing cab roll angle . 33

vi
ABSTRACT 

This report documents the updated performance characteristics of NASA 

Langley Research Center's (LaRC) Cockpit Motion Base (CMB) after recent 

revisions that were made to its inner-loop, feedback control law. The 

modifications to the control law will be briefly described. The performance of 

the CMB will be presented. A short graphical comparison to the previous 

control law can be found in the appendix of this report. The revised controller 

will be shown to yield reduced parasitic accelerations with respect to the 

previous controller. Metrics based on the AGARD Advisory Report No. 144 are 

used to assess the overall system performance due to its recent control algorithm 

modification. This report also documents the standardized simulator test 

procedure which can be used in the future to evaluate potential updates to the 

control law. 
INTRODUCTION 

The Cockpit Motion Facility (CMF) located at the NASA Langley Research Center (LaRC) contains a 6 degreeof-freedom (6-DOF) motion platform with three intercha ngeable operational flight sim ulator cockpits, also 

referred to as cabs, as well as a crane to reposition the cabs. The Cockpit Motion Base (CMB) is operated and 

maintained by LaRC’s Simulation Development and Analysis Branch (SDAB), providing NASA, DoD and many 

other institutions and companies a state-of-the-art synerg estic motion platform for aerospace research. This report 

aims to quantif y the current opera tional performance of the motion system and dis cuss a standardized 

performance test procedure method. The procedure and t ools outlined were used in determing the performance 

characteristics presented. In addition, the motivation for the recent control la w modification is discussed. The 

characteristics of the previous controlle r, which was ac tively used on the CMB until October 2012, are provided 

in the appendix for com parison purposes only. This report will also briefly document the system architecture of 

the facility, describing the modification of commanded accel erations from a host si mulation as they are filtered, 

transformed, and finally sent to drive the platform. 

 

Figure 1: GFD interior (left) and exterior on CMB (right) 

The CMB features six 76-inch leg stroke extension low -friction hydrostatic bearing actuators in a hexapod 

configuration. The facility currently has three operati onal cabs that can be placed on the motion base for motion 

testing or operated in a fixed-based environment independent of the motion base. Cabs are transferred from their 

fixed-base locations to the motion platform via an o verhead facility crane system and lifting rig. The three c abs 

are the Research Flight Deck Simulator cab (RFD), the Generic Flight Deck Simulator cab (GFD)- seen in Figure 

1, and the Integration Flight Deck Simulator cab (IFD). There is a fourth cab dock station available for future 

acquisition of an additional cab. Figure 2 contains a graphic of the overall facility. 

2
Figure 2: CMF representation showing the CMB, available cabs, and the crane 

The performance characteristics of the m otion system are continuously being defined b y the SDAB. Throug h 

systematic testing, the branch is expen ding effort to fu rther define the current state of perf ormance as well as 

implement modifications in the softwa re to improve the performance. The basis of the perfor mance metrics and 

the subsequent determ ination of the platfor m characteristics are compiled from Reference [1] and based on 

AGARD-AR-144 [2]. AGARD-144 documents methodologies for evaluating the motion platform via time and 

frequency domain characterization techniques, along w ith assessments of accel eration noise and parasitic 

acceleration. Parasitic acceleration as defined in [2] is the resulting m easured acceleration in the “undriven” 

degrees-of-freedom occuring when accelerations are co mmanded in another “driven” degree-of-freedo m. Current 

literature also refers to this phenomena as cross-coupling, or cross-talk. 

This report defines a systematic method for measuring the dynamic qualities of si mulator motion systems. 

Numerous simulators have been evaluated with the methodolgies defined in AGARD-144, includi ng the 

SIMONA Research Simulator at Delft University of Technology [3] and the Visual Motion Simulator at NASA 

LaRC. The AGARD-144 report has also been used as the basis for asse ssments of the relationship between 

motion base performance and pilot perception[2]. 

Figure 3: Control architecture for the CMB 

3
CMB MOTION SYSTEM CONFIGURATION 

The CMB relies on an integrated netw ork for command and control. Figure 3 illustrates t he primary systems 

involved in passing motion commands and processing f eedback from the motion base. A typical research 

experiment would involve a host computer with a 6DoF aircraft simulation passing acceleration commands to the 

digital control unit (DCU). The DC U contains the di gital control law (DCL) neces sary for processing and 

commanding the desired motion base trajectory. The resulting position commands are then converted to an analog 

signal commanding two valves per leg to achieve a desired leg extension/retraction. Note that the two valves 

connected to a single leg receive id entical commands. The resulting coordinated leg position adjust ment 

transports the motion base and cab to a new translati onal and rotational orientation. Position and velocity of each 

leg are measured and fed back to the DCU/DCL as an analog signal which is then sampled at 2kHz in the internal 

feedback loop. This feed back loop is separate from the aircraft simulation providing commands from the host 

computer, which would typically be r unning at <100 Hz. The platform has a set of accumulators designed to 

sustain hydraulic system pressure by providing supplemental flow during periods of high dem and, and 

subsequently to recharge while the extra pressure is not needed. 

All measurements provided in this paper will be defined re lative to a simulator reference frame which is depicted 

in Figure 4. The remaining degrees-of-freedom are the typical rol l, pitch and yaw attitudes defined in the same 

manner as traditional aircraft Euler angles. This co ordinate system is defined in more detail in [ 4]. Also, note 

that the general orientation of the six hydraulic actuators is also shown in Figure 4. 

Figure 4: Cab-axes coordinate frame 

The motion centroid excursion limits in the previously defined simulator coordinate are shown in Table 1. These 

limits are representative of the absolute position limits of the system. It should be noted, however, that m ultiple 

extremities of the excursion envelope are not realistically attainable at the same time due to geometric constraints. 

DIGITAL CONTROL LAW 

The signal to noise ratio of a motion base system, as suggested in [2], dictates the operati onal capabilities of a 

motion base simulator. As with all nonl inear mechanical systems of this type, some level of off-axis acceleration 

noise occurs naturally and degrades the operational cap abilities. The control law of the CMB was recently 

modified to reduce the presence of pa rasitic accelerations. The recent revision to the m otion base’s inner-loop 

control focused on reducing t he CMB’s off-axis acceler ation noise otherwise known as parasitic accelerations. 

The resulting revision was implemented to promote as much symmetry in the leg performance as possible. 

4
Table 1: DoF excursion capability [3] 

 Degree of Freedom Commanded Measured Difference 

Surge (inch) 67 67.88 0.88 

-55 -53.94 -1.06 

Sway (inch) 55 54.50 -0.50 

-55 -54.50 -0.50 

Heave (inch) 41 41.30 0.30 

-41 -40.00 -1.00 

Yaw (degree) 38 36.27 -1.73 

-38 -36.52 -1.48 

Pitch (degree) 28 27.30 -0.70 

-25 -25.30 0.30 

Roll (degree) 28 28.10 0.10 

-28 -28.00 0.00 

CMB Parasitic Accelerations- Past and Present 

Parasitic acceleration is defined relative to the sim ulator reference frame, however, for analy sis, it often 

convenient to analyze individual leg responses with positive or negative excursion defi ned relative to the 

orientation of the legs. Therefore, the resulting axes are constantly moving as the legs extend and retract. Figure 5 

shows the time history of a run where the co mmanded extension of a sing le leg was a constant frequenc y 

sinusoid, while the commanded extensions of the five other legs were zero. The top subplot in Figure 5 shows the 

commanded motion of the single leg, and the resulting unc ommanded movement of the remaining five legs. As 

differential piston pressure is continously measured and piston area is known, the forces in each leg are computed 

for the same time series of data and plotted in the botto m subplot. For an approximately 17,000 lb (8.5 to n) cab 

such as the GFD, the required forces to simply prevent the five “resting” legs from moving are quite significant. 

Leaving the leg position perspective, Figure 6 shows simulator reference frame heave position commanded in the 

form of a co nstant frequency sinusoidal wave with a +/- 1 inch peak am plitude. The resulting commanded 

positions in the illustrate the real-world impact of the parasitic accelerations. Uncommanded motion in the sway 

and surge degrees of freedom are clearly noticeable, in addition to m uch smaller di splacements in all three 

rotational degree of freedoms, which are seen in the zoomed snapshot. 

5
Figure 5: Uncommanded motion and residual motion for 0.45 inch- 4Hz sinusoid Leg 1 position command. 

Figure 6: Uncommanded motion during 1 inch- 2Hz sinusoid level heave command 

6
Control Law Modification 

CMB testing showed that not all legs, and m ore specifically their valves, were perform ing equally. Figure 7 

demonstrates the velocit y output of each of the s ix legs for both extend a nd retract with respect to valve 

command, where valve command repre sents the input to each leg. The valv e command for each leg is passed to 

two valves, as each leg utilizes two val ves, resulting in twice the total leg velocity for each maneuver. While the 

performance of the six legs closely match in extensi on- positive valve co mmands, there is a wider r ange in 

performance of the legs d uring retraction- negative valve commands. The valves were tested to +/- 80% (green 

region) and linearly extrapolated to generate the higher valve command percentages (pink region). 

Figure 7: Leg target-velocity test results 

To address this occurence, a gain was added in the contro l law to the retract command of the five faster r etracting 

legs to reduce the co mmanded valve position and slow their retraction to the retract velocit y of the slowest leg. 

The resulting modification resulted in reduced parasitic accelerations. Figure 8 presents the response of the CMB 

with the GFD cab during banked m otion. To assess the par asitic acceleration in the sway axis, the motion base 

was banked to several orie ntations with sinusoidal accelerations commanded in cab-axis heave. The commanded 

simulator reference frame pure heave motion consisted of constant frequency sinusoids ranging from 0.5Hz to 

4Hz, in incre ments of 0.5Hz, splined together afte r two cy cles at ea ch frequency. The measured simulator 

reference frame heave and sway accelerations in Figure 8 illustrate the l evels of parasitic (sway in this example) 

accelerations across s everal frequencies for both the pr evious and new production control laws. Significant 

reduction in the parasitic sway accelerations was ac hieved. The overall performance of t he new production 

controller is presented in the following section. For co mparison, the performance characteristics of the previous 

controller are provided in the Appendix. 

7
Figure 8: Parasitic accelerations during increasing banked heave oscillations

8
CMB PERFORMANCE AND COMPARISONS 

The motion base performance is presented mostly from a linear analysis perspective. All results represent 

the performance of the motion base with the GFD cab. The additional cabs, the IFD and RFD, have 

different masses and weight distributions- Table 2. Note that the CG informa tion is provided in body , 

specifically simulator, reference fra me. Despite th ese differences, the performance of the motion base 

with these cabs is expected to be comparable. 

PAYLOAD MASS CHARACTERISTICS from NASTRAN

GFD Properties: 

Total Mass +33342.31570 (lbm)

CG Position x =    ‐2.90     y =    ‐0.11      z =   ‐31.20 (in)

CG Force x =     0.00      y =     0.00      z = ‐33342.26 (lbf)

CG Moment x =     0.00      y =     0.00      z =     0.00 (lbf‐in)

Inertia Moments (CG) Ixx =    +433077.8   Iyy =    +424668.4     Izz =    +666520.0   (lbf‐in‐sec^2)

Inertia Products (CG) Ixy =       +433.6       Iyz =       ‐126.8          Izx =      +5275.6     (lbf‐in‐sec^2)

RFD Properties: 

Total Mass +36329.90904 (lbm)

CG Position x =    ‐3.47      y =    ‐0.19      z =   ‐41.58 (in)

CG Force x =     0.00      y =     0.00      z = ‐36329.85 (lbf)

CG Moment x =     0.00      y =     0.00      z =     0.00 (lbf‐in)

Inertia Moments (CG) Ixx =    +595489.7    Iyy =    +588245.7      Izz =    +735953.7 (lbf‐in‐sec^2)

Inertia Products (CG) Ixy =      ‐1008.8        Iyz =       +107.5          Izx =     +17987.6 (lbf‐in‐sec^2)

IFD Properties: 

Total Mass +36641.73343 (lbm)

CG Position x =    ‐4.35      y =    ‐0.44      z =   ‐41.60 (in)

CG Force x =     0.00       y =     0.00      z = ‐36641.67 (lbf)

CG Moment x =     0.00       y =     0.00      z =     0.00 (lbf‐in)

Inertia Moments (CG) Ixx =    +595191.9    Iyy =    +588097.2       Izz =    +735931.1 (lbf‐in‐sec^2)

Inertia Products (CG) Ixy =       +807.5        Iyz =       +395.6           Izx =     +18097.5 (lbf‐in‐sec^2)

Table 2. Payload Mass Characteristics 

The characterization effort focuses on the transport delay, or time delay, between simulation command 

and first measured motion was det ected, as well a s the frequency response in all 6DoFs. Becaus e this 

recent control law modification aimed to reduce the pa rasitic accelerations, the frequency response of the 

off-axis accelerations are also presented. 

Transport Delay 

The transport delay of a single leg on the motion base wa s measured using SIMES, a si mulator 

instrumentation measurement system, as approximately 25 +/- 8ms. The r esults are depicted in Figure 9. 

9
The four signals in this fig ure represent both the position and velocity feedback signal on the leg, as well 

as the two separate valve commands supporting that leg, ISVA- Valve Command A and ISVB- Valve 

Command B. ISV(A&B) are the servovalve drive current s measured across an internal shunt resistor. 

The test was conducted with a step co mmanded directly from the DCU and pr ocessed by taking 

measurements of first valve m ovement (position and velocity ). Measuring first motion of the phy sical 

motion platform is of course limited by sensor sensitivity which has not been quantified or included. 

Figure 9: System latency test using SIMES with 10kHz sampling rate- 20 sec step input to Leg 5. 

Performance Test Input Signals 

Linear analysis of CMB/G FD’s dynamic response was performed with comma nded frequency sweeps. 

Sinusoidal input signals a re preferred for perform ance analysis over im pulses, steps and ram ps due to 

their similarity to normal aircraft control inceptor inputs [4, 5]. 

݂௜ሺݐሻ ൌ ݂଴ሺݐ௜ሻ൅൤௙೔൫௧೑൯ି௙బ

௧೑

(1)																																																																						ݐ ൨

The frequency sweep, or chirp signal, used for testin g is shown in Figure 10 and given by Equation 1 [7]. 

Note that the bottom subplot of this figure is a zoomed snapshot of the acceleration command. The chirp 

signal was designed to minimize unnecessary violent accel erations on the sy stem across the t ested 

frequency ranges. For this reason, extensive testing with constant frequency sinusoids sweeping through 

a broad range of frequencies was rejected. This chir p signal is relatively short in duration. However, the 

results show adequate information for frequency domain analysis. The chirp signal co mmands simulator 

reference frame position, velocity, and acceleration. The chirp signal linearly sweeps through frequencies 

0.05 Hz, 	݂଴ሺݐ௜ሻ, to 7 Hz, ݂௜൫ݐ௙൯,	 at 10ms timesteps. 

10
Figure 10: Time history of commanded frequency sweep 

The signal was designed to command constant accel eration. To minimize integration errors, the original 

acceleration command was construct ed at 10,000 Hz and then twice integrated to derive the 

corresponding velocity and position commands. These signals were then do wnsampled to 100 Hz to 

achieve the typical command signal frequency. The chirp signal is co mposed of two s egments. The first 

segment is com posed of lower frequencies to re duce the excursion associated with low -frequency, 

constant-acceleration commands. The first half cycle of each segment is linearly faded-in to reduce initial 

transients. It should be noted, that while these linear techniques adequately capture the performance of the 

motion base, there are limitations. Linear frequency domain analysis does not fully represent the sy stem 

harmonics or other nonlinear dynamics of the system. A primary aspect of the system that is not captured 

is the fluctuation in performance as the sy stem hydraulic pressure de creases as a r esult of the 

accumulators being depleted. The Power Spectral Density (PSD) semilogarithmic plots in Figure 11 show 

the resulting power of the commanded position and acceleration in decibels across the tested frequenc y 

range. In the bottom subplot, note that the drop in power at 1 Hz is most likely due to the windowing 

around the d eadzone of the splined seg ments of th e signal du ring the Fou rier Transform, and not 

necessicarily a lack of frequency content in the chirp at that frequency. 

11
Figure 11: Power Spectral Density of commanded frequency sweep 

Performance Characteristics 

The performance of the CMB with t he GFD cab a nd the new production control law is presented in 

Figure 12 through 17. In both the previous and new production control laws, the CMB/GFD achieves a 

bandwidth approximately to 3 Hz in m ost degrees of freedom, with phase lag clearly being the lim iting 

factor. The rotational DoFs exhibit a performance roll-off in magnitude as frequency increases. The heave 

axis behaves in a similar manner while also being th e least responsive translation DoF. Surge and sway 

exhibit appreciable overshoot at higher frequencies, approximately 4 Hz, before sharply falling off i n 

response around 5Hz. The revised controller does sacri fice some phase margin in all DoFs. However, the 

closed-loop dynamics of the m otion base are not si gnificantly changed in nature. The Bode plots 

comparing the controllers can be found in the Appendix. 

12
Figure 12: Bode Plot for Production Controller - Heave 

Figure 13: Bode Plot for Production Controller - Sway 

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Heave Chirp - Bode Plot 

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Sway Chirp - Bode Plot 

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

13
Figure 14: Bode Plot for Production Controller – Surge 

Figure 15: Bode Plot for Production Controller – Pitch 

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Surge Chirp - Bode Plot

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Pitch Chirp - Bode Plot 

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

14
Figure 16: Bode Plot for Production Controller – Roll 

Figure 17: Bode Plot for Production Controller – Yaw 

The previous linear analysis techniques we re extended to as sess the cur rent levels of parasitic 

acceleration, as this was the motivation for the recent control law modification. The Bode plots in Figures 

10-1 100 -15

-10

-5

0

5

Amplitude (dB)

Roll Chirp - Bode Plot 

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

10-1 100 -15

-10

-5

0

5

Amplitude (dB)

Yaw Chirp - Bode Plot

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

15
18 through 23 show the response in all 6 DoFs to a chirp signal co mmanded in a single DoF. Thus, the y 

represent a total measured response of the parasitic acceleration present in the off-axes. The parasitic 

accelerations clearly increase with increasing frequency . Moreover, a strong linear correlation with 

parasitic acceleration and f requency is apparent. Note that neither of the plot axes are linear, so it is not 

immediately apparent in these plots. This linear can tr end can be seen seen in Figure 42 of the Appendix. 

This 3D figure was generated fro m testing done with the previous controller and has y et to be duplicated 

with the current production controller. However, the sa me trend is expected. Nonlinearities, primarily in 

the form of apparent harmonics, contribute substantially to t he parasitic accelerations in the off-axes. 

Again, these nonlinearities and their effects cannot be seen in any form of linear anaylsis. 

Figure 18: Production Controller - Parasitic Acceleration from Commanded Heave (dB) 

10-1 100 101

-100

-80

-60

-40

-20

0

20

Frequency (Hz)

Magnitude (dB)

Production Controller- Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

16
Figure 19: Production Controller - Parasitic Acceleration from Commanded Pitch (dB) 

Figure 20: Production Controller - Parasitic Acceleration from Commanded Roll (dB) 

10-1 100 101

-60

-40

-20

0

20

40

60

80

Frequency (Hz)

Magnitude (dB)

Production Controller- Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

10-1 100 101

-60

-40

-20

0

20

40

60

Frequency (Hz)

Magnitude (dB)

Production Controller- Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

17
Figure 21: Production Controller - Parasitic Acceleration from Commanded Surge (dB) 

Figure 22: Production Controller - Parasitic Acceleration from Commanded Yaw (dB) 

10-1 100 101

-100

-80

-60

-40

-20

0

20

40

60

Frequency (Hz)

Magnitude (dB)

Production Controller- Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

10-1 100 101

-60

-40

-20

0

20

40

60

Frequency (Hz)

Magnitude (dB)

Production Controller- Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

18
Figure 23: Production Controller - Parasitic Acceleration from Commanded Sway (dB) 

Additional information regarding accumulator and pump limits, plotted in the frequency domain, gives 

more insight into the dynamic operational limits of the platf orm. Approximate sustained m otion 

capabilities are depicted in Figures 24-26. The CMB/GFD response to commanded heave and sway 

accelerations is shown in Figure 24 a nd Figure 25 respectivel y. Figure 26 illustrates t he limits for 

commanded heave accelerations with a 15 degree bank angle. The latter figure illustrates the limits during 

15 degrees of bank on the off-axis. As previousl y established, the current closed loop response of t he 

platform peaks near 5 Hz before fallin g off swiftly . More information regarding the oper ational limits 

with regards to CMB safety devices can be found in [9]. 

10-1 100 101

-120

-100

-80

-60

-40

-20

0

20

Frequency (Hz)

Magnitude (dB)

Production Controller- Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

19
Figure 24: Operational Limits (Heave) 

Figure 25: Operational Limits (Sway) 

20
Figure 26: Operational Limits (Banked Heave) 

CMF STANDARDIZED TESTING PROTOCOL 

This section details testin g protocols and analysis tools to be used to periodi cally to validate the motion 

dynamics of the Cockpit Motion Base. These tools are also intended to provide comparable performance 

characteristic definitions in the event of future CMF modifications. A standard set of sinusoidal inputs for 

frequency domain analy sis will be defined and described. Testing protocols using the CMF Host Test 

Interface to drive the motion base with sinusoidal si gnals will be i ntroduced. Processing scri pts, written 

in MATLAB and designed to illustrate the performance in a consistent manner, will be described. The 

performance results can be co mpared with past CM B/GFD performance data for periodi c validation of 

proper performance or clear evaluation of performance improvement. 

 

Standard Spectral Inputs 

Frequency sweeps and banked sinusoi dal acceleration commands are used for m otion validation and are 

available as comma separated value (.csv) files. 

Chirp Inputs 

The frequency sweeps, or chirps, are used to gene rate Bode plots with inform ation on the dynam ic 

response of the motion base from its nominal orientation – defined as the position of the m otion base at 

the completion of its “hold” state in the DCU. Th e frequency sweeps are available for all 6 sim ulator 

reference frame DoF. As discussed earlier, the frequency sweep was designed to minimize unnecessary 

violent accelerations on the system across the tested frequency ranges. 

21
Banked, Sinusoidal Inputs 

The banked, sinusoidal signals are us ed to assess the uncommanded acceleration in the lateral axis a s a 

result of commanded vertical acceler ations. This si gnal consists of four cy cles of const ant-frequency 

sinusoids ranging from 1Hz to 5Hz, in increm ents of 0.5Hz, splined together with si mulator reference 

frame angular offsets from 0° to 13.5°, in 1.5° increments in the cab’s roll-axis. 

The input file commands translational displacements in inches and rotational displacements in radians. 

The commands are inertial/earth axes, NOT cab-axes. Therefo re the input files will command the 

necessary combination of inertial axis heave and sway position to achieve pure, simulator reference frame 

heave commands when the cab is banked. This m ovement of combined sway and heave was coined as 

‘sweave’. More si mply put, when the cab is ro lled and pure heave is co mmanded in the si mulator 

reference frame, the required input is combined sway and heave inertally. Figure 27 shows how this type 

of input looks in simulator reference frame. The bottom subplot is a zoomed-in snapshot of the 20 second 

splined segments seen in the top subplot. 

Figure 27: Sample sweave-spline time history 

0 20 40 60 80 100 120 140 160 180 200

-5

0

5

Commanded Heave

 Position (in)

Cab-Axis Coordinate Frame

0 20 40 60 80 100 120 140 160 180 200

0

5

10

15

Commanded Bank

 Angle (degrees)

0 2 4 6 8 10 12 14 16 18 20

-5

0

5

Commanded Heave

 Position (in)

Time(s)

22
Standard Test Procedure 

The provided test signals m ust be commanded via t he host test i nterface at 100Hz. Six frequency sweep 

files are provided, for eval uation of all 6DoFs of the CMB. Two banked, sinusoidal signals are provided 

for evaluation of the u ncommanded lateral acceleration in with left and right angular offsets. DVS data 

must be recorded for all runs so that it can be used for post-processing. 

The GFD i s the recommended cab for testing as th e post-processing tools will provide an i mmediate 

comparison with past GFD data, however the RFD or the IFD can also be used. 

Standard Post-Processing Tools 

Automated Processing Scripts 

Two post-processing scri pts will generate a majority the results in this report for co mparison of the 

current motion base performance to any future upda tes or retesting after calibrations. There is a 

‘readme.txt’ file with basic instructions and descriptions provided with the software toolset. 

Chirp Analyzer User Steps 

1. Add the folder containing your DVS files of the frequency sweeps to your MATLAB path. This 

can be done within MATLAB by clicking “File” “Set Path” and browsing to the directory in 

the resulting pop-up window. 

2. Change MATLAB directory to the folder in which the ‘ChirpAnalyzer.m’ script is located. 

3. Open ‘ChirpAnalyzer.m’ editor file. 

4. Declare the filenames of the six DVS output files from the frequency sweeps and save the m.file. 

5. Click “Run” 

Graphical User Interface (GUI) 

GUI User Steps 

1. Repeat Step 1 from Section 0. 

2. Change MATLAB directory to the folder in which SDAB_GUIv1 is located (...\Post Processing 

Scripts\SDAB GUI). 

3. Open SDAB_GUIv1.m. 

4. Run SDAB_GUIv1.m. 

5. Click “Load DVS DATA” (once). 

6. Use pop window to select the desired DVS output data file for plotting anal ysis. Note: You may 

need to specify “All Files” in th e “Files of Type” drop down m enu to see a .csv file. This data 

will take approximately a minute to load into the GUI. 

7. Click on a parameter to the left to plot. 

8. Change plot settings via button panels on the right. 

23
Figure 28: GUI - Post Processing Tool 

Additional Functions 

‘bode_plot’ – Plots the Po wer Spectral Density or Bode plot of the time domain signals sampled at the 

specified sampling frequency 

‘plotter’ - Plotting function called by various several scripts 

CONSIDERATIONS 

Future improvements may be realizable fro m both software and hardware modifications. The following 

areas of improvement are briefly introduced for awareness only. 

The nature of the parasitic acceleration demonstrates repeatability and linear relationships with increasing 

frequency of commanded accelerations. The control law could potentially be revised to include a feedforward correction from a model of t he measured parasitic accelerations. T his and other control law 

improvements are continuously under evaluation by the SDAB. 

Several modifications to the hardware could also potentially lead to improved bandwidth and reductions 

in parasitic acceler ation. A large catwalk was added to the platform for acce ssibility which ultim ately 

adds a large mass at a long lever arm from the cen troid of the platform. Any reduction or the entire 

removal of the platform would likely result in a quantifiable improvement in motion bandwidth and likely 

parasitic acceleration. 

24
Figure 29: CMB valve performance 

Valve performance from a testbed assessment indicates a current valve bandwidth of <4 Hz at 45°. The 

results of this test are presented in Figure 29. Increasing pump capacity may provide an opportunity to 

further increase motion bandwidth and allow for more control authority to further reduce parasiti c 

acceleration. Quantification of such improvements is also currently under evaluation by the SDAB. 

CONCLUSIONS 

The current, closed-loop performance of the CMB/GF D was presented as it exists with th e production 

control law. The new production contr oller showed a clear reduction in parasitic acceleration with 

minimal impact to closed-loop perform ance. Operational limits characterizing the motion envelope were 

documented. This performance cha racterization is a vital tool for future researcher s and experiments 

requiring accurate replicat ion of commanded m otion. Th e standardized testing protocol allows for the 

reproduction of these results in a consistent manner for periodic re-evaluation of the CMB as well as after 

any future system modifications, both hardware and software. 

25
BIBLIOGRAPHY 

[1] B. R. Bryant, CMF Motion Base Control Sy stem Derived Requirements. Version A., Ham pton, 

VA: NASA Langley Research Center. Airbone Systems Competency. Systems and Development Branch, 

February 5, 2001. 

[2] “Dynamic Characteristics of Flig ht Simulator Motion Systems,” Advisory Group for Aerospace 

Research and Development, North Atlantic Treaty Organization, num ber AGARD-AR-144, AGARD 

Advisory Report, 1979 

[3] B. N. Tomlinson, “Simulator Motion Characteristics and Perceptual Fidelity, A Progress Report,” 

AGARD Conference Proceedings Num ber 408, Flight Simulation, 7 Rue A ncelle, 922200 Neuill y sur 

Seine, France, pages 6A- 1--6A-12, Advisory Group for Aerospace R esearch and Development, North 

Atlantic Treaty Organization, 1986. 

[4] Robert J. Telban, Weim in Wu and Frank M. Cardullo, Motion Cu eing Algorithm Development: 

Initial Investigation and R edesign of the Algorith ms, NASA Langley Research Center, Ham pton (VA), 

number NASA-CR-2000-209863, Contractor Report, 2000. 

[5] L. Gupton, PRT for GFD Motion Shakedown (Test Plans 3.1 - 3. 5 and 3.10). Safety Interlocks 

and DOF Performance Evaluation, October 6, 2008. 

[6] W.R. Berkouwer, O. Stroo sma, M.M. van Paassen, M. Mulder and J.A. Muld er, “Measuring the 

Performance of the SIMONA Research Simulator's Motion System,” Proceedings of the AIAA Modeling 

and Simulation Technologies Conference, San Francisco, California, Aug. 15-18, 2005. 

[7] P. M. T. Zaal, F. M. Nieuwenhuizen, M. Mulder and M. M. va n Paassen, "Maximum Likelihood 

Estimation of Multi-Modal Pilot Cont rol Behavior in a Target-Following T ask," IEEE International 

Conference on Systems, Man and Cybernetics, Singapore, Oct. 12-15 2008. 

[8] Mathworks, "R2012a Documentation -> DSP System Toolbox -> Blocks -> Sources - > "Chirp"," 

Mathworks. Product Documentation., 2012. 

[9] L. E. Gupton, R. B. Bry ant and D. J. Ca rrelli, "Evaluating the Perfo rmance of the NASA L aRC 

CMF Motion Base Safety Devices," Proceedings of the AIAA Modeling and Sim ulation Technologies 

Conference, Keystone, Colorado, August 21-24, 2006. 

[10] "Integrated Intelligent Flight Deck T echnologies (2006-2010). Facilities: Cockpit Motion 

Faciility (CMF)," [Online]. Available: http://www.aeronautics.nasa.gov/avsafe/iifd/cmf.htm. [Accessed 

08 August 2012]. 

26
APPENDIX 

Comparison of Production Controller to Previous Controller 

Figure 30:Bode Plot Comparison – Heave 

Figure 31: Bode Plot Comparison – Sway 

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Heave Chirp - Bode Plot 

Production 3Hz Control Law

Previous Control Law

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

Production 3Hz Control Law

Previous Control Law

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Sway Chirp - Bode Plot 

Production 3Hz Control Law

Previous Control Law

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

Production 3Hz Control Law

Previous Control Law

27
Figure 32: Bode Plot Comparison – Surge 

Figure 33: Bode Plot Comparison – Pitch 

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Surge Chirp - Bode Plot

Production 3Hz Control Law

Previous Control Law

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

Production 3Hz Control Law

Previous Control Law

10-1 100 -15

-10

-5

0

5

Amplitude(dB)

Pitch Chirp - Bode Plot 

Production 3Hz Control Law

Previous Control Law

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

Production 3Hz Control Law

Previous Control Law

28
Figure 34: Bode Plot Comparison – Roll 

Figure 35: Bode Plot Comparison – Yaw 

10-1 100

-15

-10

-5

0

5

Amplitude (dB)

Roll Chirp - Bode Plot 

Production 3Hz Control Law

Previous Control Law

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

Production 3Hz Control Law

Previous Control Law

10-1 100 -15

-10

-5

0

5

Amplitude (dB)

Yaw Chirp - Bode Plot

Production 3Hz Control Law

Previous Control Law

10-1 100

-150

-100

-50

0

Frequency (Hz)

Phase (deg)

Production 3Hz Control Law

Previous Control Law

29
Figure 36: Previous Controller - Parasitic Acceleration from Commanded Heave (dB) 

Figure 37: Previous Controller - Parasitic Acceleration from Commanded Pitch (dB) 

10-1 100 101

-120

-100

-80

-60

-40

-20

0

20

Frequency (Hz)

Magnitude (dB)

Previous Controller - Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

10-1 100 101

-60

-40

-20

0

20

40

60

80

Frequency (Hz)

Magnitude (dB)

Previous Controller - Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

30
Figure 38: Previous Controller - Parasitic Acceleration from Commanded Roll (dB) 

Figure 39: Previous Controller - Parasitic Acceleration from Commanded Surge (dB) 

10-1 100 101

-60

-40

-20

0

20

40

60

80

Frequency (Hz)

Magnitude (dB)

Previous Controller - Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

10-1 100 101

-120

-100

-80

-60

-40

-20

0

20

Frequency (Hz)

Magnitude (dB)

Previous Controller - Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

31
Figure 40: Previous Controller - Parasitic Acceleration from Commanded Sway (dB) 

Figure 41: Previous Controller - Parasitic Acceleration from Commanded Yaw (dB) 

10-1 100 101

-100

-80

-60

-40

-20

0

20

Frequency (Hz)

Magnitude (dB)

Previous Controller - Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

10-1 100 101

-60

-40

-20

0

20

40

60

Frequency (Hz)

Magnitude (dB)

Previous Controller - Cross Coupling

Heave

Pitch

Roll

Surge

Yaw

Sway

32
Figure 42: Previous Controller – Cross-coupled acceleration gains by increasing cab roll angle 

0

5

10

15

1

2

3

4

5

0

0.1

0.2

0.3

0.4

0.5

Cab Roll Angle (deg) Frequency (Hz)

Amplitude Ratio

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

33
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Current Performance Characteristics of NASA Langley Research Center's 

Cockpit Motion Base and Standardized Test Procedure for Future 

Performance Characterization 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Cowen, Brandon; Stringer, Mary T.; Hutchinson, Brian K.; Davidson, Paul 

C.; Gupton, Lawrence E.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

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 REPORT NUMBER

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NASA

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Availability: NASA CASI (443) 757-5802

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14. ABSTRACT

This report documents the updated performance characteristics of NASA Langley Research Center's (LaRC) Cockpit Motion 

Base (CMB) after recent revisions that were made to its inner-loop, feedback control law. The modifications to the control law 

will be briefly described. The performance of the Cockpit Motion Facility (CMF) will be presented. A short graphical 

comparison to the previous control law can be found in the appendix of this report. The revised controller will be shown to 

yield reduced parasitic accelerations with respect to the previous controller. Metrics based on the AGARD Advisory Report 

No. 144 are used to assess the overall system performance due to its recent control algorithm modification. This report also 

documents the standardized simulator test procedure which can be used in the future to evaluate potential updates to the 

control law. 

15. SUBJECT TERMS

Control Theory; Motion Base; Simulation

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